The term semi-finished products denotes the general concept of prefabricated shapes made from raw materials, such as sheet metal, bars, tubes, and coils. In metal working, semi-finished products constitute by far the most common delivery form for feedstocks made of metals and plastics.
A typical characteristic of semi-finished products is that they are not, in general, used in the original dimension or size. The first processing step typically involves a blank, from which the required material section is severed, using a suitable separating method (such as cutting).
Two manufacturing methods, in particular, are known for the production of semi-finished products, which is to say metal injection molding (MIM) and hot isostatic pressing (HIP). Both of these two methods have the advantage associated with the powder-metallurgical production of components or semi-finished products, in so much as the need to finish the components is low.
Metal injection molding is a method for producing minute and small metallic parts. The method combines two known manufacturing technologies which actually have little to do with each other, these being plastic injection molding and metal sintering. The MIM method comprises four partial steps, consisting of spray mixing of the molding compound, injection molding, removing the binder, and sintering. In the first step, a suitable, very fine metal powder is mixed with a binder to form a moldable starting material. Contrary to conventional pressing and sintering, in which densities of 90% of the theoretical material density are typically achieved, the metal injection molding method achieves densities between 96% and 100% of the theoretical material density. In this way, material properties are advantageously attained which largely correspond to those of parts manufactured from solid metal.
Furthermore, when modified, the MIM method can also be used to produce particularly porous semi-finished products by employing spacers. The porosity can advantageously be set between 10 and 80% by volume.
Since, in general, the components or semi-finished products manufactured by way of the MIM method do not require any metal finishing, the more difficult a material usually is to process, the greater the advantages of the metal injection molding method are. This method is therefore particularly suited to stainless steels, soft magnetic alloys, iron/nickel materials, as well as tool steels and special alloys, such as nickel-titanium shape memory alloys.
Shape memory alloys are metals which return to their original shape after deformation, once they are heated to a certain temperature. In the process, they can develop significant forces. Possible applications for shape memory alloys include micromanipulators and robot actuators, which can imitate the smooth movements of human muscles. In reinforced concrete structures, sensors made of shape memory alloys can be used, for example, to detect cracks in the concrete or corrosion in the steel reinforcement bars and to counteract internal stresses.
Previously, shape memory alloys that are based on the intermetallic phase NiTi have preferably been produced by a fusion metallurgy process. Conventional shaping by machining of metallurgically-fused semi-finished products made of NiTi materials, however, is limited because, in the martensitic state, the alloys exhibit a high elongation at break and therefore have poor machining properties. As a result, the finishing of components produced by way of fusion-metallurgy processes involves high time expenditure and high tooling wear.
Previously, employing the metal injection molding method for NiTi shape memory alloys frequently failed because there was a need for, not only an economical production process, but also an end product having the low impurity levels required for shape memory properties. Thus, a challenge in the manufacture of NiTi components by way of metal injection molding (MIM) is that of minimizing the oxygen and carbon contents to as great an extent as possible. High impurity levels result in reduced sintering activity for the metal powders and worsen the material properties in the sintered component (due to embrittlement or the like). In addition, another important requirement for producing NiTi shape memory alloys is the precise and reproducible adjustment of the alloy composition. Even minor variations in the composition produce considerable variations in the characteristic properties (such as the transformation temperatures).
During production of the powder, in general, particles having a generally large particle size distribution are obtained, including a portion of coarser particles. Gas atomization by which the powder is obtained is presently known to produce mean particle sizes d50 as small as approximately 50 μm or even smaller.
While the use of particularly fine starting powders has the advantage, in terms of process engineering, that they exhibit excellent moldability, they are generally also associated with the risk of higher impurity levels resulting from the crucible material that is used, which has proven particularly disadvantageous for NiTi shape memory alloys comprising a third alloying element.
As a result, producing the powder by way of crucible-free (impurity-free) melting, using induction and atomization according to the EIGA (electrode induction melting gas atomization) process, is suited for particularly pure and low-impurity powders, such as those which are required for the production of semi-finished products from a shape memory alloy. However, the disadvantage is that the particle fraction of the powder produced in this way having a particle diameter larger than 45 μm usually constitutes more than 65% by weight. Separating the required fine fraction, and failing to use the separated coarse fraction, as was conventional, therefore made the use of this material for the production of semi-finished products by way of the MIM method very expensive.
For applications requiring particularly fine particles, these particles can frequently only be obtained by separating the coarse fraction from the powder that has been produced. For example, separating cuts of d97 between 2 and 120 μm can be attained by using fine sifters.
The second method for producing semi-finished products is hot isostatic pressing (HIP). This method is a development in the manufacturing engineering industry, wherein powders and solids, in particular ceramics and metals, are hot-pressed and sintered at the same time. During hot isostatic pressing (HIP), powdery or porous materials are uniformly compressed entirely without binders by applying high temperatures (lower than the melting temperature of the material) and isostatic pressure of several hundred MPa. For this method, for example, use is made of finely atomized materials from the melting bath, which ensure high-quality microstructures, resulting in a high degree of freedom in terms of selection of the alloy. Inert gases are used as the pressure transfer medium during hot isostatic pressing. Powders or highly porous materials are encapsulated in the process, while components having closed pores can also be compressed by hot isostatic pressing without being encapsulated. This results in structures that are homogeneous to an extent that is almost impossible to achieve using other methods. Components produced by way of an HIP method are extremely dense (up to 100% of theoretical density) and have isotropic properties. Advantageously, a composite component produced from different materials can be achieved with this method.
A major disadvantage of this technique is that it is very complex and therefore associated with very high manufacturing costs, particularly when low dimensional tolerances are required. The primary field of application is the redensification of sintered, metallic and ceramic workpieces for the aerospace industry, the automotive industry, or medical implants.